Monday, January 28, 2013

1 SPI Bit per Disk Bit Achieved!

I figured I would try to get started on disk writing today, even though I doubt I can finish it before the end of the month. While I was working on it, I got side-tracked and re-visited my previous attempt to reduce the number of bits I have to send via SPI to represent a bit on disk. I put this aside after claiming success by reducing it from 4 to 2 SPI bits per disk bit at 500MHz. I was playing around with 1 bit per disk bit at 300Khz, and I was getting some errors, but it seemed close. At this speed, the read_pulse is slightly longer than the expected 1usec and the time between the start of read_pulses is shorter than the average 4usec. Then I tried 250KHz and everything just worked! At 250KHz, the read pulse is long at 2usec, but the time between the start of read_pulses is the correct 4usec. This only works due to my previous discovery of ANDing the SPI MOSI and the SPI clock lines to produce a shortened read_pulse. This probably won't work on all Apple IIs, and it may be too slow to handle disk writes, so I will keep this feature as an option for now. Unfortunately, it did not fix my latency issue, even though I am sending less data per sector. However, for the first time I can send 2 disk sectors per USB transfer, now that transfer size is down. This reduces the ~4000usec delay to once every 2 sectors. Maybe the libftdi mailing list has some input on this issue.

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